1. Technical Field
The field relates to vortex cannons and more particularly to vortex cannons employing pulse detonation engines.
2. Description of the Problem
Pulse detonation engines have attracted recent research interest as potential sources of thrust for aerial vehicles among other applications. The interest in pulse detonation engines has been driven by the potential gains in thermodynamic efficiency from detonation as opposed to combustion of fuel and oxidizer. The chief factor underlying these potential efficiency gains is that the detonations generate a supersonic pressure wave front instead of the subsonic expansion associated with deflagration reactions supported by air-breathing gas turbine engines. The flame front in pulse detonation engines can propagate at speeds between 2,000 and 4,000 meters per second, well in excess of the speed of sound in the atmosphere. Application of pulse detonation engines to propulsion systems has been constrained by the inadequacy of conventional materials to handle the high heat and pressure levels generated by high frequency detonation cycling.
One characteristic of pulse detonation engines is that they produce a high level of noise. For a source of thrust this can be a disadvantage, at least where the vehicle is intended to operate in the atmosphere. But it led to recognition of the possible application of pulse detonation engines as sources of acoustic energy as described in U.S. Pat. No. 3,064,753. Galley, in U.S. Pat. No. 4,356,753, expanded on the teachings of the '753 patent. Galley adapted a “rocket engine type combustion chamber” to achieve sound “similar to that of a pipe organ and resonator.” In Galley's musical instrument a fuel/oxygen mix was admitted to a combustion chamber for ignition by a spark plug. The sound resulting from ignition of this mixture was described as “explosive.” Galley appears to have contemplated having the spark frequency, which was controllable, correspond to the intended frequency of the output sound. Galley did not specify a fuel type nor make explicit whether the character of the combustion process was deflagration or detonation though detonation seems likelier. The high operational frequencies asserted for the device, up to 2000 Hz., are high compared to pulse detonation engines when adapted for the generation of sufficient pressure to provide effective thrust for vehicle. Thrust generating pulse detonation engines seem generally to have been limited to about 25 Hz. The Galley device was strictly an acoustic application.
Pulse detonation engines have also found application in so called “hail” cannons which are a type of vortex cannon. Wiering a and Holleman, writing in the Meteorologische Zeitschrift, 15, issue 3, June 2006, noted that hail cannons were initially developed by Albert Stiger in 1896. Early hail cannons used a gunpowder charge while contemporary hail cannons use an acetylene/air mix in a combustion chamber. The pulse generated by detonation of the mix is directed through a compression throat into a conical barrel to generate a ring vortex at the muzzle. Effects of the ring vortex may be seen in a characteristic “smoke ring” which is actually moisture condensed out from the atmosphere. A typical firing rate for these devices is about once every four seconds although some references indicate rates up to 1 Hz. While the effectiveness of these devices for preventing the formation of or destroying hail is largely discredited the devices can be used to produce a substantial ring vortex which can propagate up to 100 meters from the barrel muzzle. A demonstration of a modern hail cannon may be seen at <<http://www.youtube.com/watch?v=lyAyd4WnvhU>>.
Ring vortices are examples of circular fluid flow occurring in a ring or toroidal region. Put more technically, a ring vortex is fluid motion in a circle around a line where the line curves back on itself in a closed loop. Fluid speed in the vortex is inversely proportional to distance from the line. Where unconstrained a ring vortex formed in the atmosphere will travel along a path centered in the closed loop and perpendicular to the plane of the closed loop. The greater the internal speed of the fluid caught in a ring vortex the greater the duration, stability and propagation speed the ring vortex will exhibit. Fluid velocity in the ring vortices produced by hail cannons can be supersonic which explains why the formations propagate for up to or more than 100 meters. Assuming a calm atmosphere the path of travel of a ring vortex will be straighter the more the closed loop approaches a true circle and as the velocity profile through the vortex is more consistent for cross sections of the vortex anywhere along the curved line. A vortex cannon will “shoot straight” if the gas pulse produced in the cannon exits the muzzle of the cannon in a plane which is perpendicular to the center line of the cannon “barrel.”